Measuring torsional distortion with an easily applied clip device (saw)

ABSTRACT

A device for measuring torsional distortion of a body comprises first and second clip portions each having a central part and two legs depending from the central part, each leg having adjacent the free end thereof a groove or projection for engaging a respective projection or groove ( 14 ) provided in the body ( 16 ) to mount the clip portion the body rotationally fastened therewith. A bridge interconnects the clip portion. The bridge is less stiff than the clip portions whereby relative rotational displacement of the clip portions caused by torsional distortion of the body will cause proportional deflection of the bridge. Means, for example a SAW device ( 15 ), are provided for measuring the deflection of the bridge to provide an indication of the torsional distortion which produces the deflection of the bridge. Also ( 20 ) for the interior all of a hollow body.

BACKGROUND

1. Technical Field

This invention relates to measuring torsional distortion, and in thepreferred embodiment provides a method and device for measuring thedistortion in a body which is subject to applied torque. The torsionaldistortion measured may be used, in known manner, to determine themagnitude of the torque to which the body is subject.

2. Description of Related Art

It has been known for many years to measure the torque to which a body(for example a shaft) is subject by applying one or more strain gaugesto the surface of the body and interrogating the output of the straingauges to provide an indication of torque. In recent years, torquemeasurement techniques have been substantially improved by theintroduction of Surface Acoustic Wave (“SAW”) based torque measurementdevices (see for example EP-A-05 18900). Such devices have a number ofwell recognized advantages including high sensitivity and the ability toprovide torque measurements without the need for hard wired connectionsto the SAW device.

Up until now, the conventional technique for mounting torque measuringdevices has consisted of preparing the surface of the body whosetorsional distortion is to be measured and securing the torque measuringdevice to the prepared surface. The preparation may consist of acleaning process subsequent to which the torque measuring device issecured to the cleaned surface by a bonding technique—for exampleadhesive bonding or soldering. However, many torque measuring devicesrequire a flat surface upon which to be mounted and accordingly thepreparation may include the step of providing a flat surface, forexample by machining, on the body whose torsional distortion is to bemeasured, prior to securing the torque measuring device to the preparedflat. These techniques are well recognized as being disadvantageousbecause of the time and cost involved and because they limit thepossibility of retrofitting a torque measuring device to an existingshaft. Further, the mounting of the torque measuring device requiresboth skilled labor and well defined manufacturing conditions and isaccordingly difficult to carry out under field conditions. As a resultit is generally necessary to apply torque measuring devices in a factoryenvironment before the body whose torsional distortion is to be measuredis mounted in its final use position. As a result the torque measuringdevice is susceptible to accidental damage during the period betweenmounting of the torque measuring device and the final positioning of thebody whose torsional distortion is to be measured. Finally, existingtechniques require the permanent mounting of the torque measuring deviceon the body whose torsional distortion is to be measured. As a result,if the torque measuring device fails, substantial additional work willbe involved in terms of removing the faulty unit before a replacementunit can be installed.

Accordingly, there exists a need for a method and apparatus formeasuring torsional distortion of a body which may easily be applied tothe body without extensive preparation of the body. The need also arisesfor such a device which can be applied under field as well as factoryconditions and which can be applied by skilled, semi-skilled andunskilled labor. A need also arises for such a device which can readilybe removed and replaced in the event that it becomes faulty.

SUMMARY OF THE INVENTION

In accordance with the first aspect of the present invention a devicefor measuring torsional distortion of a body comprises: first and secondclip portions each having a central part and two legs depending from thecentral part, each leg having adjacent the free end thereof a groove ora projection for engaging a respective projection or groove provided onor in the body to mount to the clip portion on the body rotationallyfast therewith; a bridge interconnecting the clip portions, the bridgebeing less stiff than the clip portions whereby relative rotationaldisplacement of the clip portions caused by torsional distortion of thebody will cause proportional deflection of the bridge; and measuringmeans for measuring the deflection of the bridge.

The preferred embodiment of the invention makes use of clip portionswhich, using a ridge-and-groove mounting technique, can be mountedrotationally fast with a body whose torsional distortion is to bemeasured. The clip portions are interconnected by a bridge portion. Thebridge portion is less stiff than the clip portions with the result thatwhen the body undergoes torsional distortion, and as a result the clipportions rotate relative to each other about the axis of the appliedtorsional force, a corresponding proportional deflection of the bridgewill occur. The exact mode of deflection of the bridge is complex, butis essentially torsion, which is measured by sensing the tensile andcompressive direct strain components of the maximum shear strain, whichacts at the center of the bridge at ±45° to the longitudinal axis.

In the preferred embodiment of the invention the means for measuringdeflection of the bridge is mounted on the bridge itself and may, forexample, comprise a SAW device. If a SAW device is used, suitableexcitation and analysis equipment may be located near, but not connectedto the SAW device. Such an arrangement is particularly desirable sincethe measuring device does not need to be hard wired and can accordinglysimply be clipped into position for use. If the device becomes faulty ora different range of torsional measurement is required the device cansimply be removed by unclipping and replaced with an alternative device.

To facilitate ready application and removal the clips are preferablyresiliently deformable so that they can be snapped into and out ofengagement with the body.

A particularly simple embodiment of the invention may be fabricated frommetal as an integral structure. The structure may be formed by anysuitable technique, for example by forming a stamped or machined blankand bending it into the required shape, or by investment casting, or bymachining from solid.

In a particular preferred arrangement the ridges are each generalV-shaped in transverse cross-section and are formed by substantiallyflat sides connected by a curved ridge. The curved ridge is,conveniently, a portion of the surface of a cylinder which blends intothe flat sides. The grooves are also preferably V-shaped in transversecross-section and have flat sides which either meet at a line or areconnected by a curved surface of a smaller radius than that of theridges. The included angle of the ridges is preferably less than that ofthe grooves. With such an arrangement, the ridges will engage thegrooves along lines of contract located where the curved ridge blendsinto the flat sides of the ridges. Such mounting arrangement gives aparticularly stable, back-lash free and well defined mounting giving ahigh level of mechanical stability to the arrangement and accurateinterpretation of the deflection of the bridge. In the preferredembodiment of the invention the included angle of the ridges issubstantially 60° and the included angle of grooves is substantially90°.

The invention may be embodied either for use on the exterior surface ofa body, in which case the projections are preferably formed on theinwardly facing side of the legs for engagement with the grooves formedin the exterior surface of the body, or for use in a hollow body, inwhich case the ridges are preferably formed on the outwardly placingsides of the legs for engagement within grooves provided in the walls ofa passage provided in the body.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptionof preferred embodiments thereof; given by way of example only,reference being had to the accompanying drawings wherein:

FIG. 1 is a schematic perspective view of a first embodiment of theinvention;

FIG. 2 is an end view of the embodiment of FIG. 1;

FIG. 3 is a detail of the area marked “A” on FIG. 2;

FIG. 4 shows the embodiment of FIGS. 1-3 mounted on a shaft;

FIG. 5 is a view corresponding to FIG. 1 but showing an embodiment ofthe invention for use in a hollow body;

FIG. 6 is an end view of the device of FIG. 5;

FIG. 7 is a detailed view of the area marked “B” in FIG. 6; and

FIG. 8 is a schematic view showing the device of FIG. 5 mounted in ahollow shaft.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to FIGS. 1-3 the illustrated measuring device 1comprises two clip portions 2,3 interconnected by a bridge portion 4.The illustrated device is an integral metal structure and may be formedby any suitable method, for example by bending a stamped or machinedblank, by investment casting, or by machining from solid. The structureis designed such that the bridge 4 is relatively less stiff than theclip portions 2,3 with the result that any distortion of the devicecaused by mutual relative rotation of the clip portions 2,3 will cause;predominantly, deflection in the bridge 4.

Preferably, the entire structure is resilient so that it will adopt theillustrated configuration in the absence of imposed forces.

Each clip portion 2,3 comprises a central part 5 and two legs 6,7. Eachleg has, at the free end thereof, a projection 8. The projections are inthe form of ridges which are generally V-shaped in transversecross-section and are made up of substantially flat sides 9,10interconnected by a curved ridge 11. The ridge 11 is preferably in theform of a portion of the surface of the cylinder and the flat surfaces9,10 are tangential to the notional surface of the cylinder of which theridge 11 forms part.

Cut-outs 12,13 are preferably provided at the junctures between the legs6,7 and the central part 5 with the result that the legs 6,7 may beresiliently deflected away from each other to allow the projection 8 tobe snap fitted into respective grooves provided on the exterior surfaceof a body, for example a shaft. FIG. 4 illustrates the measuring deviceof FIGS. 1-3 so mounted. The grooves 14 in which the projections 8 sit(only two are illustrated in FIG. 4 but there are a corresponding two onthe other side of the shaft 16) are also generally V-shaped intransverse cross-section with generally flat sides. In the case of thegrooves, however, the flat sides either meet at a line or at a curvedsurface having a smaller radius than the radius of the curved portion 11of the ridges. As a result, the ridges are precisely located in thegrooves and make contact with the grooves along two lines correspondinggenerally to the lines at which the flat sides 9,10 meet the curvedridge 11.

The required grooves in the shaft can conveniently be formed byindentation using a suitable tool. Forming the grooves in this manner isrelatively simple and the pressing action will result in some protectiveresidual compressive stresses in the zone of the grooves to deterfatigue crack initiation.

While FIG. 4 illustrates only a single device mounted on a shaft 16 itshould be appreciated that a plurality of similar devices may be mountedon the shaft utilizing a common set of grooves or adjacent grooves. Forexample, two devices may be mounted on diametrically opposite sides ofthe shaft to ensure dynamic balance. In this case, the grooves 14 willbe located somewhat offset from a diametric plane through the shaft witha small portion of shaft surface located between each pair of grooves.

Measuring means are provided for measuring the deflection of the bridgecaused by relative rotation of the clip portions produced by torsionaldistortion of the body. The measuring means may be of any suitable type,but in the preferred embodiment comprise one or more SAW devices 15secured to the bridge to measure deflection thereof. SAW devices have anumber of well recognized advantages in the measuring of deflection. Oneparticular advantage of SAW devices is that they are entirely passive(that is they do not require a battery to be connected to them in orderto operate) and they can be interrogated by radio signals. Accordingly,provided that suitable excitation and interpretation circuitry isprovided somewhere adjacent the device, the device as illustrated inFIG. 1 may be used without any hard wiring to it. This is particularlydesirable when a device is used on a shaft which is subject tocontinuous rotation.

Referring now to FIGS. 5-7 an alternative embodiment of the invention isshown. In this case, the measuring device 20 is intended for use in theinterior of a hollow body, for example a tubular shaft. The measuringdevice includes two clip portions 21,22 each of which comprises acentral part 23 and two legs 24. The clip portions 21,22 areinterconnected by a bridge 25 which is relatively less stiff than theclip portions. In this case, each leg 24 is provided with a projectionin the form of a ridge 26 located on the exterior surface thereof forengagement with a corresponding grooves provided in the wall of a hollowbody. The profile of the ridges 26 and their associated grooves is asdescribed above with reference to FIGS. 1-4.

FIG. 8 illustrates the measuring device 20 of FIGS. 5-7 mountedinternally in a hollow tubular shaft 27.

The device 20 is provided with measuring means 28 for measuring thedeflection of the bridge 25. The measuring means are preferably asdescribed above with reference to FIGS. 1-3.

The invention, especially when embodied with a SAW device for measuringdeflection of the bridge, forms a self-contained unit which can readilybe clipped into engagement with a body, for example a shaft. Althoughthe body in question requires preparation by the formation of grooves,this is a relatively simple and inexpensive operation and grooves canreadily be provided as part of the standard manufacturing techniquesassociated with the body in question. Fitting of the measuring device tothe body is a simple matter of clipping the device into position. Thiscan readily be carried out under field conditions by semi-skilled labor.Further, a faulty measuring device can readily be removed and replacedby service staff simply by unclipping one measuring device and replacingit with another. The particular arrangement of ridge and groovedescribed for locating the device ensures stable and precise location ofthe device leading to a mechanically stable arrangement and accurateinterpretation of the output of the bridge deflection measuring device.

1. A device for measuring torsional distortion of a body comprising:first and second clip portions each having a central part and two legsdepending from the central part, each leg having adjacent a free endthereof at least one of a groove or a projection for engaging at leastone of a respective projection or groove provided on or in the body tomount the clip portion on the body rotationally fast therewith; a bridgeinterconnecting the clip portions, the bridge being less stiff than theclip portions whereby relative rotational displacement of the clipportions caused by torsional distortion of the body will causeproportional deflection of the bridge; and a measuring means formeasuring the deflection of the bridge.
 2. The measuring deviceaccording to claim 1, wherein the measuring means includes means forsensing tensile and compressive direct strain components of a maximumshear strain, which acts at the center of the bridge at ±45° to alongitudinal axis.
 3. The measuring device according to claim 1, whereinthe means for measuring deflection of the bridge is mounted on thebridge.
 4. The measuring device according to claim 2, wherein the meansfor measuring deflection is a SAW device.
 5. The measuring deviceaccording to claim 1, wherein the clip portions and the bridge are anintegral structure.
 6. The measuring device according to claim 1,wherein the projections are ridges which are generally V-shaped intransverse cross-section and have substantially flat sides connected toeach other by a curved ridge.
 7. The measuring device according to claim6, wherein the grooves are generally V-shaped in transversecross-section and have generally flat sides.
 8. The measuring deviceaccording to claim 7, wherein an included angle of the ridges is lessthan an included angle of the grooves, and the grooves and ridges are seshaped so that each ridge engages each groove along two lines ofcontacts spaced from a base of the groove.
 9. The measuring deviceaccording to claim 8, wherein the included angle of the ridges issubstantially 60° and the included angle of the grooves is substantially90°.
 10. The measuring device according to claim 1, in which the clipportions are resiliently deformable to allow the clip portions to besnapped into engagement with the grooves.
 11. The measuring deviceaccording to claim 1, wherein the projections are on the legs and facedinwardly so that the clip portions may be mounted on grooves provided onthe external surface of a shaft.
 12. The measuring device according toclaim 1, wherein the projections are on the legs and face outwardly sothat the clip portions may engage grooves provided in the interior wallof a hollow body.
 13. The measuring device according to claim 1, whereinthe clip portions and the bridge comprise an integral metal structure.